Move the tax bill. Who really pays?

A buyer’s card states what a crate is worth to them; a seller’s card states its cost. Each student submits one private bid or ask. When collection closes, the projector shows the anonymous curves made by those accepted orders. These are submitted prices, not a claim that everyone truthfully reported the value on their card.

The next two comparisons are deliberately hypothetical. The engine keeps the original order book fixed and computes one outcome with the per-trade tax collected from sellers, then another with it collected from buyers. The legal assignment changes the posted price, while the traded quantity, all-in buyer payment and net seller receipt agree across the two taxed cases whenever trade occurs. Students can compare both with the original no-tax market.

That is a narrower claim than saying a second classroom market would produce the same result. Students might change their bids, learn from the first round or react to how the tax is described. Use the held-fixed comparison to identify the accounting and incentive question before discussing those behavioral possibilities. This activity does not measure demand or supply elasticity or predict the incidence of a real tax.

Try the screen-and-phone preview with fictional classmates, who submit their orders through the same compiler and runtime as the lesson. You run the projector’s steps and play one student’s phone, entering that student’s own order. The classmates are not real students, and their orders are not presented as optimal strategies. Run it again with a different order to see how your trade and the curves change.